If the Virtual Server is on one Route Domain, and the pool members are on another, then you must disable "strict-isolation" for the VS RD in order for traffic to traverse between the Route Domains. If the VS and pool members are on the same Route Domain, then nothing special needs to be done in that regard.
The BIG-IP will forward to a pool member if and only if the pool members are in a netblock that also contains a BIG-IP self-IP (in the same Route Domain) or if the BIG-IP has a route to the pool members (also in the same Route Domain). Moreover, the BIG-IP will not send traffic if ARP fails for the next hop (i.e., the pool member if the member is directly connected, or the next hop gateway if a route is followed).
Do you have monitors associated with the pool members? If so, are they working (returning an "up" status)? Are the VS and pool members in the same Route Domain? If not, have you disabled strict-isolation?
Have you verified that ARP is returning successfully for the next hop (using
tmsh show net arp
and
tcpdump -nni 0.0 arp
)?